What law of motion that states that an object will remain at rest or move in a straight line at a constant speed unless it is acted upon by a force

From center station, a train departs every 30 minutes on the fast line and a train departs every 50 minutes on the state line. if two trains depart from center station at 8: 00 a.m., one on each of the two lines, what is the next time that two trains, one on each line, will depart at the same time?

The speed of light in a material is 0.50 c. what is the critical angle of a light ray at the interface between the material and a vacuum?

What is the single most important equation in all of physics?